1. Field of the Invention
The present invention relates in general to a control apparatus for an oil pump provided on a vehicle, and more particularly to techniques for improving characteristics of a rise of a hydraulic pressure which is generated by one of two oil pumps driven by respective two different drive power sources upon switching from the other oil pump to the above-indicated one oil pump.
2. Discussion of Prior Art
There is known a control apparatus for controlling two oil pumps that are selectively driven by respective two different drive power sources such that one of the two oil pumps is operated while the other oil pump is held at rest. For example, a motor vehicle is provided with a mechanical oil pump driven by an engine, and an electric oil pump driven by an electric energy supplied from a battery. In this type of vehicle, the mechanical oil pump is operated during an operation of the engine, and the electric oil pump is operated while the engine is at rest. JP-2001-41067A discloses techniques for improving characteristics of a rise of a hydraulic pressure generated by the electric oil pump, by starting the electric oil pump prior to stopping of the engine upon switching from the mechanical oil pump to the electric oil pump. JP-2006-161837A discloses techniques for improving the characteristics of the hydraulic pressure rise of the electric oil pump, by operating the electric oil pump for a predetermined short time prior to stopping of the engine, for thereby stirring and removing air staying at and around an oil inlet of the electric oil pump, upon switching from the mechanical oil pump to the electric oil pump.
In oil pump systems wherein the mechanical oil pump and the electric oil pump are selectively driven, as disclosed in the above-identified publications JP-2001-41067A and JP-2006-161837A, air tends to easily stay at and around the oil inlet of the mechanical oil pump while the electric oil pump is operated to generate the hydraulic pressure. If the mechanical oil pump is driven by the engine in the presence of the air, the air is sucked into the mechanical oil pump, giving rise to a risk of a delayed rise of the hydraulic pressure generated by the mechanical oil pump. Although JP-2006-161838A discloses the techniques for improving the characteristics of the hydraulic pressure rise of the electric oil pump by removing the air staying at and around the oil inlet upon switching from the mechanical oil pump to the electric oil pump, there are known no techniques for removing the air staying at and around the mechanical oil pump upon switching from the electric oil pump to the mechanical oil pump.